This partially reproduced for me.
I did not see my session use go to 100%. I did however get:
> API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"You're out of extra usage. Add more at claude.ai/settings/usage and keep going."},"request_id":"redacted"}
yeah, this smells like a bug in their (dumb) usage segmentation.
For example, there is a distinction of what is classified as extra-usage-billed VS extra-usage-enabled. As a long time claude user, I can assure you they are different things: to use Sonnet[1m] you are required to have extra-usage enabled, but it won't actually bill it unless you are out of quota. Surprisingly, you can use Opus[1m] without extra-usage enabled (!!!).